1. Field of the Invention
The present Invention relates to a battery pack including a chargeable battery, such as a nickel-cadmium battery, a nickel-hydrogen battery, or a lithium-ion battery, used as the power source for a portable power tool.
2. Description of the Related Art
Recently, secondary batteries, such as nickel-cadmium batteries, nickel-hydrogen batteries, and lithium-ion batteries, have increased their capacity and greatly improved charge/discharge performance when charged and discharged with large current. These high-performance secondary batteries are used as the power source of high-load machines, such as cordless power tools (referred to as simply “power tools” hereinafter). Secondary batteries used in power tools are normally in the form of a battery pack that includes a battery made from battery cells connected in series by, for example, a connection plate and used as the power source for high-load devices, enabling the high-load devices to be cordless. A high-performance chargeable battery can generate a great deal of heat because it discharges large currents and is also charged using large currents. This heat can reduce the life of the battery. Also, when the battery is made from a number of cells connected in series, the lower rated capacity cells in the battery can easily become overcharged or over-discharged. When the lower rated capacity cells are repeatedly overcharged and over-discharged, the life of only the lower rated capacity cells is shortened.
The charging characteristic of a battery will be described with reference to FIG. 1. As shown in FIG. 1, the voltage V, temperature T, and internal pressure P of the battery gradually increase from start of charge until the battery is almost fully charged. However, when the battery is almost fully charged at time F, the voltage V, temperature T, and internal pressure P of the battery rapidly increase. With this feature in mind, whether or not the battery is fully charged is determined by detecting the rapid change in the battery's voltage V and temperature T when the battery is near full charge. If charging of the battery continues beyond the full charge time F, then the battery becomes overcharged in region O.
FIG. 2 shows a charging characteristic of lower rated capacity cells. As shown in FIG. 2, the voltage VL, temperature TL, and internal pressure PL of the lower rated capacity cells gradually increase from start of charge similar to the curves shown in FIG. 1. However, the lower rated capacity cells of the battery become fully charged at a time FL, which is earlier than when the other cells become fully charged. As a result, the charge condition of the lower rated capacity cells has already entered the overcharged region O before the battery is detected to be fully charged at time F. Because the lower rated capacity cell is further charged after its charged condition enters the overcharged region, temperature TL and internal pressure PL of the lower rated capacity cell increases to the point where the lower rated capacity cell deteriorates. When the battery is repeatedly discharged and charged, the lower rated capacity cell is repeatedly overcharged and over-discharged. The lower rated capacity cell can leak electrolyte as a result. Internal impedance can also increases. In association with this, the capacity of the lower rated capacity cell rapidly decreases, potentially leading to an internal short circuit or disconnection. These can shorten the life of the battery pack.
FIG. 3 shows the cycle life of a chargeable battery in two situations A, B. In situation A the battery is repeatedly discharged 100% and charged to 100% of its capacity. In situation B, the battery is discharged 80% and charged to 80%. That is, in situation B discharge is stopped before the battery is fully discharged, that is, when the battery is only 80% discharged, and charging is stopped before a full charge is achieved, that is, when the battery has been charged to only 80% of its capacity. As described above, the battery's life is much shorter in situation A when the battery is discharged and charged 100% than in situation B when discharged and charged only partially.
In order to increase the life of batteries used in, for example, hybrid electric vehicles (HEV), the partial discharge and charge method shown in FIG. 3 is implemented to prevent the batteries from over-discharging and overcharging.
However, in order to implement the partial discharge and charge method, the battery voltage and temperature of all of the cells in the battery, or of a number of cell groups in the battery, need to be monitored. This requires complicated control circuitry. A battery pack that includes such complicated circuitry is too expensive for incorporation into products for every day use.